This invention relates to an improved wet milling process for refining corn into useful products, in which recovery of proteins such as gluten is enhanced.
Corn kernels contain starch, protein, water, fiber, and other substances which can be separated to make various useful products. Gluten is one component of the kernel, and contains the majority of the protein that is present in corn, thus making it a desirable component for animal feed.
In order to refine the corn into end products such as starch, oil, and feed, a corn wet milling process is used. In general, kernels of corn are steeped in an aqueous solution under controlled conditions to soften the kernels and facilitate separation of the kernels' components. After steeping, the aqueous solution, referred to as steepwater, is drawn off. The corn kernel, swollen as a result of the steeping, is then coarse-milled to allow removal of the germ. Oil is removed from the germ and refined to make corn oil. The remainder of the germ is dried to form corn germ meal, or it may be used as an ingredient in corn gluten feed.
After the germ is removed, the remainder of the kernel is milled again to pulverize endosperm particles while leaving fibrous material nearly intact. Fiber is separated from the starch and gluten by screening. The fiber is then combined with the dried remains of the steepwater to make corn gluten feed.
The starch and gluten are then separated, in what is often referred to as the primary starch separation step, and the gluten is dried to form corn gluten meal. The primary starch separation is typically done by centrifugation, and produces a gluten-containing stream that typically comprises about 3-5% (by weight) total solids. This stream is usually sent through a gluten thickener centrifuge. This centrifugation produces a stream that is primarily water and a concentrated gluten stream (e.g., about 15% solids). Unfortunately, a significant amount of gluten is lost in this centrifugation step (i.e., is in the stream that is primarily water). The concentrated gluten stream is then usually sent to a gluten vacuum filter, further concentrating it, e.g. to about 38-43% dry solids. This concentrated stream is then sent to a gluten drier, to remove most of the remaining water (e.g., to about 90-95% dry solids). The dry end product is corn gluten meal.
Some of the starch is dried and may optionally be chemically modified before being sold to the food, paper, or textile industries. Corn sweeteners or ethanol are produced from the remaining starch.
Thus two of the main animal feed products of the corn wet milling process are corn gluten feed and corn gluten meal. Corn gluten feed is a medium protein, medium energy product. It is used in feeds or concentrates for cattle, poultry, and swine. It is commonly sold containing about 20% protein. Corn gluten meal is a high protein, high energy product. It is a valuable source of methionine to complement other commonly used protein sources in animal feed. Also, its high xanthophyll content makes it particularly valuable as an efficient pigmenting ingredient in poultry feeds. It is a particularly excellent feed ingredient for cattle, because it provides a high level of rumen-protected protein.
Because of its superior characteristics as animal feed, corn gluten meal can be sold for a significantly higher price than corn gluten feed. Therefore, corn refiners prefer to maximize production of corn gluten meal relative to corn gluten feed. In a typical wet milling process, significant amounts of gluten remain in end products other than corn gluten meal. However, this type of protein is too small to be recovered by centrifugation or conventional filtration.
Efficient means of concentrating and recovering the proteins from various aqueous process streams are important to the overall economics of the corn wet milling process.
Microfiltration and nanofiltration are techniques that have been used in the past in some starch-related applications. For example, European patent application 0,452,238 discloses using a nanofiltration membrane to filter a starch slurry, passing the dextrose in the slurry through the membrane while retaining the di-and trisaccharides, thereby producing a glucose syrup which is about 95% dextrose and 5% di- and trisaccharides. European patent application 0,176,621 discloses a process for obtaining glucose from thinned starch. The process includes a separation step that produces a glucose-enriched stream and a glucose-depleted stream. Membrane-based separation is disclosed as one suitable separation technique.
There is a long-standing need for enhanced recovery of proteins such as gluten in corn wet milling processes.